Hemophilia
Hemophilia is a disease characterized by slowed blood clotting and can be caused by genetic or environmental abnormalities or stresses (1). The two clotting factors affected in hemophilia are factors VIII and IX. Deficiencies in either of the two factors are known to cause the most severe problems in the blood clotting cascade (2). Hemophilia A, B, and acquired will be discussed in terms of how their respective factors are affected, and how these changes affect the function of the factor. Hemophilia A Hemophilia A is caused by a mutation in Factor VIII and can be split into two categories. The condition where produced Factor VIII is functional but unstable or secreted at a reduced rate is called Type I, and Type II refers to when the enzyme itself is not functioning properly, but is still secreted normally (3). To differentiate between types I and II, the activity and antigen concentration are compared. If activity: antigen is less than 0.7, the defect is determined to be Type II (3). A few causes of more severe Type II have been connected to a few specific amino acid missense mutations, specifically those required for activation of the enzyme, like Arg391→Cys/His, Ser392→Pro/Leu, and Arg1708→Cys or involved in binding of the ligands, specifically changes with N-glycosylation (3). Since Factor VIII is critical for amplifying the activation of Factor X (2), any changes in the function or secretion of the Factor VIII are critical, so both types I and II can be considered serious diseases. Hemophilia B Hemophilia B is caused by a mutation in the Factor IX gene found on the X chromosome (4). Under normal conditions, Factor IX circulates in the blood as a single-chain serine protease in the zymogen form (4). It is stimulated by the intrinsic pathway in the presence of calcium ions to convert to its active, two-chained form which has a gla and catalytic domain (4). Many mutations on the gene have been identified with varying severities. One study focused particularly on the effects of two different mutations on the same amino acid residue in the Factor IX gene. When gly-317 mutated to glu, moderate bleeding was observed, while a substitution to arg produced severe bleeding (4). Even though both mutations caused the enzymes to have a decreased affinity for their normal substrate Factor X, kcat for the activation of the glu mutant increased (4). It was suggested that the differences in severity between the mutation to positively-charged arginine and negatively-charged glutamine were due to the amino acids that they mutated to. Factors IX and VIII work together to activate Factor X, so any changes that decrease the activity of Factor IX can be considered severe. Acquired Hemophilia Unlike Hemophilia A and B, acquired hemophilia is not caused by a genetic mutation but rather by an autoantibody raised against a change in the internal or external environment such as a congenital auto-immune response, cancer, or prescription drugs (5). Similar to Hemophilia A, acquired affects Factor VIII, which normally circulates in the blood with its non-covalently bound chaperone protein, von Willebrand factor (5). Specific sections of three of the six domains of Factor VIII are known to be binding sites for autoantibodies. Antibodies binding the C2 domain between amino acids 2181 and 2243 block the enzyme from phospholipid and chaperonin binding, which would decrease its stability in the bloodstream (5). Binding between amino acids 454-509 and 593 in the A2 domain inhibits binding to Factor X, and between amino acids 1804 and 1819 in the A3 domain inhibits binding to Factor IXa which slows the progression of the blood clotting cascade, particularly because Factor X is crucial for the activation of thrombin (5). Often, the antibodies are observed to have a polyclonal response, meaning that at least two of the three previously mentioned binding sites are affected, depending on if the disease is congenital or acquired after an environmental change (5). References 1. U.S. National Library of Medicine. Genetics Home Reference: Hemophilia. https://ghr.nlm.nih.gov/condition/hemophilia#genes (accessed December 6, 2018). 2. Voet, D.; Voet, G.; Pratt, C.W. Fundamentals of Biochemistry: Life at the Molecular Level, 2nd ed.; John Wiley & Sons, 2006; Box 11-4 fig. 2. 3. Kemball-Cook, G; Gomez, K. Molecular Basis of Hemophilia A. In Textbook of Hemophilia; Lee, C.A.; Berntorp, E.E.; Hoots, W.K., Eds.; Wiley: West Sussex, 2014; 3rd Ed; p 27-30. 4. Lu, Q.; Yang, L.; Manithody, C.; Wang, X.; Rezaie, A.R. Expression and Characterization of Gly-317 Variants of Factor IX Causing Variable Bleeding in Hemophilia B Patients. Biochem. Online 2015, 54, 3814-3821 DOI: 10.1021/acs.biochem.5b00270 (accessed December 6, 2018). 5. Kessler, C.M. Acquired inhibitors to factor VIII. In Textbook of Hemophilia; Lee, C.A.; Berntorp, E.E.; Hoots, W.K., Eds.; Wiley: West Sussex, 2014; 3rd Ed; p 87-88.